Embed Size (px)
Citation preview
LUND UNIVERSITY
PO Box 117221 00 Lund+46 46-222 00 00
Towards Nanowire Tandem Junction Solar Cells on Silicon
Borgström, Magnus T.; Magnusson, Martin H.; DImroth, Frank; Siefer, Gerald; Höhn, Oliver;Riel, Heike; Schmid, Heinz; Wirths, Stephan; Björk, Mikael; Åberg, Ingvar; Peijnenburg, Willie;Vijver, Martina; Tchernycheva, Maria; Piazza, Valerio; Samuelson, LarsPublished in:IEEE Journal of Photovoltaics
DOI:10.1109/JPHOTOV.2018.2816264
2018
Document Version:Peer reviewed version (aka post-print)
Link to publication
Citation for published version (APA):Borgström, M. T., Magnusson, M. H., DImroth, F., Siefer, G., Höhn, O., Riel, H., ... Samuelson, L. (2018).Towards Nanowire Tandem Junction Solar Cells on Silicon. IEEE Journal of Photovoltaics, 8(3), 733-740.https://doi.org/10.1109/JPHOTOV.2018.2816264
Creative Commons License:Unspecified
General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal
Read more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.
Download date: 11. Aug. 2020
Abstract—The development of photovoltaics as a serious means of producing renewable energy has accelerated greatly
in the last ten years, with prices for silicon-based solar cell systems dropping dramatically in the last few years. The nextgreat opportunity for photovoltaics following this competitiveness in prices will be to enhance the cell and panel efficiencies.It is quite generally seen that the most viable platform on which this should be realized will be as augmented silicon solarcells, in which a top cell will be combined with the silicon bottom cell in a tandem configuration, by which the efficiency canbe enhanced by a factor from 20% to 50%, depending on details of the approach. In this paper we report on the status ofone such approach, namely with a top cell comprised on III-V NWs, connected to the bottom silicon cell in a two-terminalor four-terminal configuration. Among the most important opportunities, we show that a substrate-free growth, calledAerotaxy, offers a radical reduction in the total price picture. Beside the description of the key technical approaches, wealso discuss the environmental issues.
Index Terms—Photovoltaics, tandem cells, nanowiress, environmental impact.
INTRODUCTION
ilicon-based photovoltaic cells are the dominant technology for terrestrial solar energy conversion. After manydecades of research and development, efficiencies have essentially saturated with the best devices measuring26.7 % in the laboratory [1, 2]. Significantly higher conversion efficiencies up to 38.8 % under one sun
illumination have been reached with multi-junction cells based on III–V semiconductors in flat plate configuration[3]. However, this technology has been too expensive for use in large area flat-plate modules, and the combination ofhigh efficiency solar cells in tandem configuration on a silicon (Si) platform has recently attracted great attention usingthin film III-V materials [4] as well as perovskites [5]. The use of III-V nanowires (NWs) allow a significant reductionin material needs and have the potential to exceed the performance of conventional devices after optimizing synthesis,and processing for materials quality, surface passivation and minimizing contact resistance. NWs offer to combinelattice mismatched materials with band gaps optimized for absorption of the solar spectrum due to reduction in latticematching requirements making it possible to grow them monolithically on silicon [6],[7]. The efficiency of NW-basedphotovoltaics is rapidly increasing [8], and references therein, with recent record efficiencies of 13.8 % for vaporliquid solid (VLS) [9] grown indium phosphide (InP) NWs [10] and 15.3 % efficiency for VLS grown gallium arsenide
Manuscript received July 24, 2017. This work was supported in part by the Swedish energy agency, Swedish research council, Knut and AliceWallenberg Foundation, Nanolund and Myfab. It was supported by the European Union’s Horizon 2020research and innovation programmeunder grant agreement No641023 (Nano-Tandem). This publication reflects only the author’s views and the funding agency is notresponsible for any use that may be made of the information it contains.
M. T. Borgström, M. H. Magnusson and L. Samuelson are with Solid State Physics/NanoLund at the Lund University, Lund, Sweden. (phone:+46 46 222 7679; fax: +46 46 222 3637); (e-mail: [email protected], [email protected], [email protected])
F. Dimroth, G. Siefer and O. Höhn are with the Fraunhofer Institute for solar energy research, Freiburg, Germany (e-mail:[email protected], [email protected], [email protected]).
H. Riel, H. Schmid and S. Wirths are with IBM Research Laboratory, Ruschlikon, Switzerland. (e-mail: [email protected],[email protected], [email protected]).
M. Björk and I. Åberg are with Sol Voltaics AB, Lund, Sweden (e-mail: [email protected], [email protected]).W. Peijnenburg and M. Vijver are with the University of Leiden, Leiden, the Netherlands. (e-mail: [email protected],
[email protected]).M. Tchernycheva and V. Piazza are with the C2N-CNRS, University Paris Sud, University Paris Saclay, Orsay, France. (e-mail:
[email protected], [email protected])
Towards Nanowire Tandem Junction Solar Cellson Silicon
Magnus T Borgström, Martin H Magnusson, Frank Dimroth Senior Member, IEEE, Gerald Siefer,Oliver Höhn, Heike Riel Senior Member IEEE, Heinz Schmid Member, IEEE, Stephan Wirths, MikaelBjörk, Ingvar Åberg Senior Member, IEEE, Willie Peijnenburg, Martina Vijver, Maria Tchernycheva,
Valerio Piazza and Lars Samuelson. Member, IEEE
S
(GaAs) NWs [11]. The current record in NW photovoltaic efficiency is 17.8 %, reported for top down etched InP NWs[12]. III–V NWs in combination with today’s silicon photovoltaic (PV) technology have the potential ofsimultaneously allowing very high performance, efficient use of materials, and low cost. The first step to multi-junction technology has been taken where InP NW PV has been integrated on silicon [13]. More importantly, opencircuit voltage addition of GaAs NW PV grown on a silicon solar cell, previously modelled[14], was recently shown[15]. For improved performance, the NW materials band gap has to be tuned for an optimal absorption of the solarspectra in combination with silicon. This involves the use of ternary NW materials like (doped) gallium indiumphosphide (GaInP) and gallium arsenide phosphide (GaAsP), which represent the high band gap segment in a tandemjunction geometry. In this contribution, we report on the concepts and most recent advances made with respect tointegrating NWs on a silicon solar cell platform. Together we pursue three main concepts for increasing the efficiencyof silicon-based PV by integration with NWs, and evaluate the environmental performance of using NWs instead ofthin films by means of a life cycle assessment (LCA) approach in which different impact categories as well as toxicityare considered.
I. NANO-TANDEM CELL CONCEPTS
Concept I: Direct growth of NWs on silicon solar cell.In this approach we use a catalyst-free growth technique to directly integrate III–V semiconducting NWs on silicon
using selective area epitaxy within nanotube templates. The benefit of this template-assisted selective epitaxy (TASE)growth mode is the ability to precisely control the crystal morphology and location of the III-V NWs, as well as todirectly grow III-Vs on many silicon crystalline orientations, even poly-silicon [16]. TASE hence enables the growthof vertical NWs on standard (001) silicon and thus the use of cost-efficient substrates. Furthermore, TASE offers anattractive way to establish epitaxial contact to silicon at the NW/silicon interface and its use as a tunnel junction [17].The use of metal organic vapor phase epitaxy (MOVPE), which is a mature technique and direct growth on silicon,allows large area deposition, which can be facilitated by holographic methods or nano-imprint lithography forpatterning.
In this approach, the silicon bottom solar cell uses a planar pn-junction with a polished front surface. Therefore,light trapping structures for increasing absorption may only be integrated on the back of the silicon substrate.
Fig. 1 Simple sketch of the three different concepts. a) Schematic of template-assisted epitaxy (TASE) and SEM image of afabricated structure, b) SEM image of epitaxially grown NWs, peeled off and held with a tweezer, intended for transfer to a Si cell.The membrane is black due to efficient light absorption in the NW membrane, c) substrate-free growth by Aerotaxy, example ofNWs grown by Aerotaxy.
Concept II: NWs synthesized on InP or GaAs substrate for peel off, transfer and substrate reuse.The benefit of this approach is that NW nucleation on native substrate is the most controlled, and the use of mature
MOVPE. Here the NWs are embedded in a polymer film after growth, peeled off from the substrate, and bonded tosilicon. Then the III–V substrate can be reused for further growth. The process of peeling off and transferring micro-NWs has successfully been shown by the Atwater group [18, 19]. The challenge is to develop the method for NWswith dimensions and frictional properties much smaller than those for micrometer diameter NWs. Economically viablepre-patterning, in this case with metal nanoparticles as catalyst for the NW growth, is essential. In concept I, the twocells are monolithically integrated, and therefore by necessity current matched, which creates performance issues sincethe color content in the sunlight varies as a function of time of the day and year. In concept II, the option is either tointegrate the NW array by forming a direct semiconductor–semiconductor contact, or to form a (semi) transparentcontact on the bottom side of the NW-containing membrane. The second case allows the top and bottom cells tooperate individually with a total of four leads out which results in an advantage not requiring current matching.. Inthis case, light-trapping structures can be integrated into the silicon front surface between the NW film and the Sibottom cell. This helps to enhance the light propagation and therefore absorption in the silicon bottom cell.
Concept III: Substrate-less NW growth: Aerotaxy.Aerotaxy was invented partly to test the fundamentals of NW growth in absence of a substrate, and to address the
high cost of producing NWs for large-area applications, especially solar cells. In Aerotaxy, NWs grow from seedparticles directly in the gas phase, and the resulting NW aerosol can be transferred into an ink for storage and furtherprocessing. One remarkable aspect of Aerotaxy is that this continuous process results in growth rates on the order of1 µm/s, which is 100–1000 times faster than MOVPE growth [20]. Fig. 2 is an illustration of the Aerotaxy process,for the case of GaAsP pn-junction NWs. The experimental, research scale, Aerotaxy equipment in use at LundUniversity yields approximately 2 mg of size-selected NWs per hour, which is equivalent to 4 NWs per µm2 on a 4’’wafer. Note that Aerotaxy needs no lithography and is inherently scalable. Similar to concept II, this approach allowsa choice of current matching or not.
Fig. 2 Schematic of the Aerotaxy process, where Au seed particles are grown into GaAs(P) NWs in a continuous flow. Thesequential growth of the intended NW device design is shown in the enlarged model NWs to the right.
A. Concept I resultsIn this approach, a high-energy band gap III-V NW -silicon tandem junction solar cell is targeted by epitaxial growth
of the III-V NW junction using TASE on top of the silicon planar cell. In order to achieve a high-energy band gapabsorption complementing that of silicon, a ternary III-V NW with a band gap of approximately 1.7 eV is required.Therefore we investigated GaAsP and InGaP directly grown on silicon. The vertical oxide templates were fabricatedby spin-coating a sacrificial carbon layer (1.5 µm thick) on the substrate, which was covered by a silicon dioxide(SiO2) layer and patterned by lithography (e.g. electron beam lithography or interference lithography, for cost reasons)and reactive ion etching. We used the patterned SiO2 layer as a hard mask to define sacrificial carbon NWs withdiameters between 100 nm and 200 nm by dry etching. To create the vertical oxide tube, we deposited SiO2
homogeneously around the NWs, after which the top part of the shell was opened and the sacrificial carbon NW wasremoved by etching. This process is suitable for large area growth and any silicon substrate orientation. Furthermore,the thermal budget of the developed process including NW growth allows for the implementation with Si photovoltaiccell fabrication.
The selective epitaxy of GaAsP and InGaP on Si(001) and Si(111) within vertical templates was explored atgrowth temperatures between 550 °C and 650 °C and the influence of V/III ratio (20 – 120) was studied. Highergrowth rates were observed for InGaP as compared to GaAsP. Using Si(001) the composition of GaAsP as well asInGaP was non-uniform under the process conditions evaluated. This was revealed by cathodoluminescence (CL)and electron dispersive x-ray (EDX) measurements using NW arrays as well as individual NWs. We ascribe theinhomogeneity to different Ga versus In and As versus P incorporation for InGaP and GaAsP, respectively on thegrowth facets. A previous study showed the formation of (111)B and {110} growth interfaces on Si(100) [21]. Thisis different in the case of Si(111) where only a nearly atomically flat, horizontal (111)B top facet is present. Wetherefore focused on Si(111) substrates, as illustrated in Fig. 3, showing an X-TEM image of an InGaP NW withsingle (111)B facet. Fig. 3 shows room temperature photoluminescence (PL) data of a NW array, with a PL areaintensity map in Fig 3b). We attribute the spots with higher intensities to non-uniform filling of the templatesleading to parasitic growth outside the template. The PL of a NW array (7x7 mm2) comprising approximately 50NWs was mapped using an excitation wavelength of 532 nm and a step size of 100-200 nm. The NWs within thearrays exhibit lengths of up to 1.5 mm, widths of 150 nm in a pitch of 1 mm and were grown at a constant V/III andgroup III gas ratio (Trimethylgallium(TMGa)/(TMGa+Trimethylindium (TMIn)) of 26 and 0.43, respectively. Thehighest growth temperature lead to the most favorable optical properties, like narrow full width at half maximum
(FWHM) and PL peak distribution in the array, as summarized in Fig. 3 c). The peak position of 1.67 eV (approx.30 % Ga) is close to the required value for efficient tandem solar cells.
Fig. 3 a) Tilted view SEM micrograph of an InGaP NW array grown on Si(111); the oxide template is stripped. b) PL intensitymap of a NW array grown at 550 °C. c) PL peak positions and FWHM for InGaP NWs grown at 550 °C, 600 °C and 650 °C. PLpeaks and FWHM were extracted from spectral maps covering NW arrays. The step size between PL spectra was 100-200 nm.Insets show the temperature dependent peak distribution
Fig. 4 shows the cross-section of a representative InGaP NW enclosed in the template with the corresponding EDXanalysis. A homogeneous in-plane composition is observed with a slight decrease of In content along the growthdirection, which can be attributed to the difference in surface diffusion length between In and Ga species, respectively,on the oxide mask. This phenomena is known in selective area epitaxy were the species fluxes are modified inproximity of the masking area. The transmission electron micrographs (TEM) show a flat NW growth surface and azinc blende (ZB) crystal with planar defects on (111) direction and misfit dislocations located at the NW-siliconinterface. These experiments suggest that InGaP with proper composition can be integrated directly on silicon usingTASE.
Fig. 4 TEM and EDX measurements of a single InGaP NW grown in a template on Si(111), showing a uniform NW geometry withflat interface to Si and flat top facet and a nearly homogeneous composition along the NW.
Nanoscale analyses are essential to probe individual NW pn-junctions in order to analyze the material quality, toassess the NW-to-NW homogeneity and to detect eventual failures. CL and electron beam induced current (EBIC)microscopy techniques allow to probe optical properties and current generation with a resolution of several tens ofnanometers.
We performed EBIC microscopy and single NW current-voltage characterization on TASE grown InGaP NWscontaining a pn-homojunction by introducing diethylzinc (DEZn) and disilane (Si2H6), respectively. As expected, we
observe a strong induced current at the position of the pn- junction (Fig. 5). An important step will be to demonstratean InGaP-Si tunnel junction. Initial results using a highly n-doped InGaP NW on degenerately p-doped Si show theappearance of a negative differential resistance, (NDR) but only after cooling to 150 K. While promising, this clearlydemands continued work to improve the tunnel junction. Eventually, surface passivation can be further improved bystripping the template oxide from the NWs and growing a higher bandgap III-V shell around the NWs.
Fig. 5 Electrical probe contacting an individual InGaP NW grown on Si. Under reverse bias, a generated EBIC current reveals theposition of the InGaP pn-junction.
B. Concept II resultsIn a first step to realize high energy band gap NWs for NW silicon tandem junction solar cells by metal organic vaporphase epitaxy, peel off and transfer, we synthesized and characterized p-doped ternary GaInP NWs aiming for a bandgap of about 1.7 eV in a pattern defined by nano imprint lithography for optimal light absorption in InP NWs [22].InGaP was chosen over GaAsP due to previous knowledge of doping. It was found that the use of DEZn for p-typedoping leads to more efficient TMGa pyrolysis, which increases the Ga content in the NWs as a function of attempteddoping [23]. The opto-electrical properties of ternary InGaP NWs grown by Au-assisted MOVPE were characterizedby CL and EBIC. CL showed that the NWs had a band gap close to 1.7 eV and a good NW-to-NW and intra-NWcompositional homogeneity. In two analyzed samples the room-temperature peak emission and its deviations were1.739 ± 0.041 eV and 1.640 ± 0.022 eV, respectively. For both samples, the variation of the peak CL energy along theNW axis was less than 20 meV. EBIC mapping allowed to directly visualize the internal field region having a widthvarying from 500 nm to 1 µm. Diffusion lengths of minority carriers around 150-200 nm were determined by analyzingthe EBIC profiles.
NW growth, peel-off and reuse of substrateIn order to make epitaxially grown NW solar cells economically viable, one requirement within the project is to maketechnology for recycling of the substrate available, diminishing substrate costs. In fig. 6, SEM images are shown of a)selective area grown NWs grown on an InP substrate b) the sample surface after the first peel-off, c) NWs grown afterrecycling the substrate twice, i.e, using the sample for growth three times. The NWs are indistinguishable, indicatingthat further recycling of the substrate is feasible.
Fig. 6 NW growth and peel-off. SEM images of a) selective area MOVPE grown NWs. 1st time of substrate use. b) substrateafter first peel-off. A hexagon of circular markers indicates the position of openings in the SiN growth mask for clarity. c)Selective area MOVPE grown NWs, 3rd time of substrate use. The scale bar for all images is 1 µm. Fig. 6 a) and 6 c) are takenunder a 30 degree angle towards the normal of the plane. Fig 6 b) is a top view image.
Regarding the quality of the NWs regrown on the used substrates, we identify the NW crystal as wurtzite. From theSEM images in fig. 6 a) and c) it can be seen that the NWs are self-similar with respect to morphology. We evaluatedthe optical quality by photoluminescence spectroscopy and observed characteristic behavior that can be explained bydifferences in twinning sequence leading to varying lengths of wurtzite and zinc blende segments in the NWs [24, 25],independent of the number of times the substrate had been used.
Cell integration of peeled-off membranesWhile substrate re-use was demonstrated using selective area grown NWs, the highest efficiency of epitaxially grownNWs have been shown by GaAs or InP NWs grown by the VLS method. Therefore, such NWs were peeled andintegrated according to the process flow of Fig. 7.
Fig. 7 The process flow for integrating peeled-off NWs into cells, ready to stack on Si cell according to Concept II. After NWgrowth (a), the NWs are embedded in a polymer and transferred to a carrier substrate (e.g. UV-release/PET tape) by peeling (b).The rear transparent conductor is added (c) and after bonding to a glass substrate, the carrier is released (d) prior to deposition ofthe front transparent conductor.
The mechanical properties of the NW-polymer-carrier system determine the ability to peel the NWs. For integration,we grew GaAs NWs on a 2” GaAs wafers using a square pattern with 500 nm pitch. One important parameter is thediameter of the NW, which together with the pitch is set by the requirements for optical absorption, but also by theneed for additional radial passivation layers [11]. To enable peeling, we had to limit the radial passivation layerthickness to 15 nm on a ~165 nm diameter core, less than the ideal 30-40 nm required for ideal passivation [11]. Thecurrent voltage curve of a 5x5 mm2 peeled NW GaAs cell with efficiency of 3.5% is shown in Fig. 8. The short circuitcurrent density (Jsc= 6.8 mA/cm2) is lower than on the best GaAs NW cells made from epitaxially grown NWs (Jsc >20 mA/cm2). In addition to the thinner than ideal passivation layer, this can be attributed to losses from the Au catalystseed particle not being removed [26]... For comparison, sister wafers with cells integrated on wafer resulted in a Jsc of7 mA/cm2 (with 15 nm passivation) and a Jsc of 12 mA/cm2 (with 40 nm passivation). Both of these wafers wereintegrated with the Au seed particle remaining. This shows that the peeling process and removing the native wafersubstrate is not the reason for the low current. The open circuit voltage was nearly identical to the open circuit voltageof cells integrated on wafer, while the fill factor was significantly lower, a result of series resistance at least in partdue to the not yet optimized rear contact.
Fig. 8 The current-voltage curve of a peeled GaAs NW cell. The test conditions are close to 1 sun, AM1.5G, and measured at roomtemperature, however the measurement was not calibrated.
Note that the high open circuit voltage observed in this cell was higher than typical. Most cells under test on severalpeeled membranes were heavily shunted. We determined that despite the appealing visual appearance of the peeledmembranes, as shown in Fig. 9 (a), the NWs in the membranes were tilted by approximately 45°, causing a narrowprocess window for the emitter contact etch, causing shunting of the pn-junction located near the top of the NW Fig.
9 (b). While the exact reason for the NW tilt is not clear, an investigation showed that such tilt is present already beforepeeling after the polymer dried out. One plausible explanation is that evaporation of solvents from the polymer mayhave resulted in stress, tilting the NWs. To address the shortcomings of the peeling method, we will instead use themembrane formation process developed for substrate-less Aerotaxy grown NWs (Concept III) also for MOVPE grownNWs, sonicated from the wafer substrate. As discussed further below, we have shown that GaAs NWs havingoptimized radial passivation layers can be nearly perfectly aligned without tilt on centimeter scale (Fig. 11). The restof the integration process remains the same, as per Fig. 7. Thus, there is a clear path to address the issues encounteredin the initial cells fabricated.
Fig. 9. (a) A peeled GaAs NW membrane, after transfer to a 2” glass carrier. (b) We found that NWs on these membranes are tilted.Subsequent dry etching damages the side-wall of the emitter and pn-junction, causing shunt leakage in many cells. Specifically,the oxide near the top of the NWs is fully etched (see circles). We can avoid this issue by combining the NWs and cell integrationconcepts from Concept II with the membrane procedure of Concept III.
C. Concept III resultsAerotaxy can produce NWs of excellent crystalline and optical quality, and we have so far demonstrated the followingmaterials and dopants: pure GaAs [20], p-GaAs (with Zn) [27], n-GaAs (with Sn) [28], pn-GaAs (Zn and Sn) [29],and undoped GaAs1-xPx in the range 0 < x < 42% [30] which is why we chose GaAsP rather than GaInP for Aerotaxy;the ideal composition for a tandem diode on Si is approximately GaAs0.75P0.25. Zn and Sn doping behave quitedifferently, as illustrated in Fig. 10. While DEZn does not have a detrimental effect on the NW growth rate, even asmall amount of TESn reduces the NW length measured after growth. It is also clear from the SEM images that theuse of TESn as a dopant precursor can give rise to parasitic radial growth, especially at the base end of the NW. Forthis reason, we are planning to evaluate n-type doping by use of dihydrogensulfide (H2S) as dopant precursor. Foraerotaxy using DEZn as p-type dopant source we note that the seed particle diameter observed after growth increaseswith dopant molar fraction used. The NW diameter does not show a corresponding increase, which most probably isa consequence of the expected changes in wetting angle due to the presence of DEZn[31, 32]. Photoluminescence andelectrical characterization confirm that the doping with Zn(Sn) results in p(n)-type NWs with carrier concentrationsaround 1–5×1019 cm-3.
Fig. 10 Top: SEM images of typical Aerotaxy NWs afterZn and Sn doping, where the dopant is introduced together with the TMGa. Bottom: Influence of dopants on the NW length. Notethat the TESn concentration is 100 x less than that of DEZn.
Aerotaxy is a promising route to low cost GaAs NW material fabrication for large-scale solar applications. However,with this approach, the control of NW position on a substrate is lost as the NW are grown continuously and directlyin the gas phase, forming a dust of randomly oriented NWs. Sol Voltaics has developed a proprietary technology toalign randomly distributed NWs, collected in a liquid solvent, into a thin membrane of ordered, vertically aligned, andoriented NWs. Fig. 11 shows a 30×30 mm2 GaAs NW membrane bonded to a glass substrate; to illustrate that this method not onlyapplies to Aerotaxy-grown NWs, these NWs are radially passivated and grown by use of MOVPE. The averagealignment and orientation yield over the membrane is 98.9 % and 99.4 % respectively with a surface density of 4.6NWs per µm2 demonstrating the technology.
Fig. 11 Left: 30×30 mm GaAs NW membrane made from a colloidal solution, bonded to a glass carrier substrate. The scale bar is10 mm. Right: Close-up view of aligned NWs in the membrane. The scale bar is 1 µm.
D. Environmental impactsWe assessed the environmental impacts of the three basic concepts developed for integrating NWs on a silicon solar
cell platform by performing a preliminary LCA. The LCA is based on evaluation of the environmental impacts fromcradle-to-gate: production of raw materials, use, disposal of materials and energy requirements to manufacture thetechnology in the laboratory. The laboratory technology is the basis for the development of scenarios and thedevelopment of a sustainability roadmap for the up-scaled commercial production of NW-based tandem solar cells.The functional unit of lab-scale production of one 4-inch NW film on silicon wafer was selected as the basis for theLCA. The system boundaries include the laboratory processes and the related material and energy inputs forproduction of one wafer for the different manufacturing routes. We quantified the system of processes for the differentmanufacturing routes and the related material and energy data, and used the EcoInvent database (version 2.2) tocompile data for the life cycle inventory of the background system. The CMLCA software developed at LeidenUniversity was used to compile the system of processes for the different manufacturing routes, the related materialand energy inputs, and the actual impact assessments.
The initial LCA showed that carbon emissions induce the majority of the environmental impacts. Carbon emissionsfor the production of one 4-inch silicon wafer at the laboratory scale equal 140 Kg CO2-eq, which is equal to 54,250Kg CO2-eq for the lab-scale production of 1 m2 of solar cell surface. Fig. 12 shows a breakdown of the carbon
emissions per process for Concept II.
Fig. 12. Carbon emissions contribution per process for Concept II (no substrate reuse, no stamp reuse), RIE = reactive ion etching.
As can be seen from Fig. 12, production of the III-V substrate, fabrication of the stamp and MOVPE processes havethe highest impacts in terms of carbon emissions. As substrate and wafer production make up more than 50 % of thetotal carbon emissions, reuse of the substrate and stamp will have an important impact in reducing carbon emissions.
Contribution analysis allows investigating which processes up-stream in the value chain contribute most to carbonemissions. Fig. 13 shows that mainly nano-imprint lithography and reactive ion etching contribute to carbonemissions.
Fig. 13: Carbon emissions contributions per process upstream in the value chain for Concept II.
The preliminary LCA for the laboratory production of one 4-inch NW silicon wafer shows that it is mainly theelectricity requirements of the processes that contribute to carbon emissions. No more than 5 % of the carbon emissionsare due to disposal of solvent mixtures and material usage has a very small impact. Given likely options for reuse ofmaterials and reduction of energy input, up-scaling of the concepts towards commercial production of the solar celltechnology will most likely induce a significant reduction of the environmental impacts.
II. CONCLUSION
Based on the research presented here, NWs offer a highly interesting opportunity in augmenting the performanceof standard silicon photovoltaic cells by adding a layer of vertically oriented III-V NWs on top of the silicon cell. Insuch a tandem cell configuration, NWs with inherently efficient light absorption can potentially enhance the silicon-only cell efficiency by more than 20% relative, as shown for thin films [1].
ACKNOWLEDGMENT
The authors greatly thank Dr. Enrique Barrigón, Gaute Otnes, Vilgaile Dagyte, Xulu Zeng, Dr. Nicklas Anttu, Prof.Mats-Erik Pistol, Yang Cheng, Irene Geiselaars, Georgios Pallas, Dr. Wondwosen Metaferia, Sudhakar Sivakumar,Dr. Magnus Heurlin, Linda Johansson, Giuliano Vescovi, Dr. David Lindgren, Christian Sundvall, Damir Asoli, Dr.Jaime Castillo-León, Nicolas Bologna, Moritz Knoedler, Jan Benick, Jan Christoph Goldschmidt and Hubert Hauserfor helpful collaboration within the Nano-Tandem project.
Prof. Magnus T. Borgström is an expert in semiconductor epitaxy and NW synthesis for electro-optical physics. He is thecoordinator of Nanoenergy research at NanoLund (h-index 35).
Dr. Martin Magnusson is leading research in aerosol-based methods to create nanoparticles and NWs directly in the gas phase,a process known as Aerotaxy (h-index 24).
Dr. Frank Dimroth is head of the department “III–V epitaxy and solar cells” at FHG has 20 years of expertise in high efficiencyIII–V multi-junction solar cell research (h-index 27).
Dr. Gerald Siefer is head of the team “III–V cell and module characterization” and is a specialist for multi-junction solar cellmeasurement and calibration (h-index 17).
Dr. Oliver Höhn is project manager at Fraunhofer ISE, with research focus on optical characterization and optical modelingof highly efficient multi junction solar cells (h-index 7).
Dr. Heike Riel is IBM Fellow, Executive Director of the IBM Research Frontiers Institute, and Director IoT Technology &Solutions. Her research interests are in nanoelectronic and optoelectronic materials and devices. (h-index 34).
Dr. Stephan Wirths is a PostDoctoral researcher at IBM. His research interests include semiconductor NW and layer growthtechniques as well as nano- and optoelectronic devices (h-index 15).
Mr. Heinz Schmid is a Senior Engineer with interests in epitaxy, device fabrication and electronic and optical devices. (h-index45).
Dr. Mikael Björk is CTO of Sol Voltaics AB. Prior to joining the company, he was a permanent research staff member at IBMResearch in Switzerland for six years. He holds several patents with many highly cited scientific papers (h-index 31).
Dr. Ingvar Åberg received a Ph.D. from MIT in 2006. He is the Director of Cell Engineering at Sol Voltaics AB, leading thecompany’s program on high efficiency photovoltaic cells based on NWs (h-index 13).
Prof. Dr. Willie Peijnenburg is professor in “Environmental Toxicology and Biodiversity”, focusing on the integration ofemerging environmental issues and (eco)toxicology (h-index 47).
Prof. Dr. Martina Vijver is assistant professor at Leiden University. Her research is focused on environmental risk assessmentand field validation of actual impacts of chemical stressors (h-index 23)
Dr. Maria Tchernycheva received her PhD in Physics from University Paris Sud in 2005. She is currently CNRS scientist with9 years of experience in NW physics and applications in charge of NanoPhotoNit group at IEF (h-index 27).
Mr. Valerio Piazza is an early stage researcher enrolled in a PhD program with focus on nanoscale characterizations and NWPV applications (h-index 1).
Prof. Lars Samuelson is recognized for his research on low-dimensional structures for basic science and applications. He haspublished >600 papers and is Founder and CSO of Glo AB, Sol Voltaics AB and Qunano AB (h-index 80).
REFERENCES
[1] M. A. Green et al., "Solar cell efficiency tables (version 50)," Progress in Photovoltaics, vol. 25, no. 7, pp. 668-676, 2017.[2] K. Masuko et al., "Achievement of More Than 25 % Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell,"
Photovoltaics, IEEE Journal of, vol. PP, no. 99, pp. 1-3, 2014.[3] M. A. Green and S. P. Bremner, "Energy conversion approaches and materials for high-efficiency photovoltaics," Nature Materials,
Review vol. 16, no. 1, pp. 23-34, 2017.[4] S. Essig et al., "Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency," Ieee Journal of Photovoltaics, vol.
6, no. 4, pp. 1012-1019, 2016.[5] K. A. Bush et al., "23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability," Nature Energy vol. 2, p.
17009, 2017.[6] T. Mårtensson et al., "Epitaxial III−V NWs on Silicon," Nano Letters, vol. 4, no. 10, pp. 1987-1990, 2004.[7] E. P. A. M. Bakkers, M. T. Borgström, and M. A. Verheijen, "Epitaxial Growth of III–V NWs on Group IV Substrates," MRS Bulletin,
vol. 32, no. 2, pp. 117-122, 2007.[8] G. Otnes and M. T. Borgstrom, "Towards high efficiency NW solar cells," Nano Today, vol. 12, pp. 31-45, 2017.[9] R. S. Wagner, Ellis, W.C., "Vapor-Liquid-Solid Mechanism of Single Crystal Growth" Applied Physics Letters, vol. 4, no. 5, pp. 89-90,
1964.[10] J. Wallentin et al., "InP NW Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit " Science, vol. 339, no.
6123, pp. 1057-60, 2013.[11] I. Aberg et al., "A GaAs NW Array Solar Cell With 15.3% Efficiency at 1 Sun," Ieee Journal of Photovoltaics, vol. 6, no. 1, pp. 185-
190, 2016.[12] D. van Dam et al., "High-Efficiency NW Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium-Tin-
Oxide Mie Scatterers," Acs Nano, vol. 10, no. 12, pp. 11414-11419, 2016.
[13] M. Heurlin et al., "Axial InP NW Tandem Junction Grown on a Silicon Substrate," Nano Letters, Article vol. 11, no. 5, pp. 2028-2031,2011.
[14] Y. Hu, M. Li, J. J. He, and R. R. LaPierre, "Current matching and efficiency optimization in a two-junction NW-on-silicon solar cell,"Nanotechnology, vol. 24, no. 6, p. 065402, 15 2013.
[15] M. Q. Yao et al., "Tandem Solar Cells Using GaAs NWs on Si: Design, Fabrication, and Observation of Voltage Addition," NanoLetters, vol. 15, no. 11, pp. 7217-7224, 2015.
[16] H. Schmid et al., "Template-assisted selective epitaxy of III-V nanoscale devices for co-planar heterogeneous integration with Si,"Applied Physics Letters, vol. 106, no. 23, 2015.
[17] M. T. Bjork et al., "Si-InAs heterojunction Esaki tunnel diodes with high current densities," Applied Physics Letters, vol. 97, no. 16,2010.
[18] A. C. Tamboli et al., "Wafer-Scale Growth of Silicon Microwire Arrays for Photovoltaics and Solar Fuel Generation," IEEE Journal ofPhotovoltaics, vol. 2, no. 3, pp. 294-297, 2012.
[19] M. D. Kelzenberg et al., "Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications," Nature Materials,vol. 9, no. 3, pp. 239-244, 2010.
[20] M. Heurlin et al., "Continuous gas-phase synthesis of NWs with tunable properties," Nature, vol. 492, no. 7427, pp. 90-94, 2012.[21] M. Borg et al., "Vertical III−V NW Device Integration on Si(100)," Nano Letters, vol. 14, no. 4, p. 1914, 2014.[22] G. Otnes et al., "Strategies to obtain pattern fidelity in NW growth from large-area surfaces patterned using nanoimprint lithography,"
Nano Research, vol. 9, no. 10, pp. 2852-2861, 2016.[23] G. Otnes, M. Heurlin, X. L. Zeng, and M. T. Borgstrom, "InxGa1-xP NW Growth Dynamics Strongly Affected by Doping Using
Diethylzinc," Nano Letters, vol. 17, no. 2, pp. 702-707, 2017.[24] J. M. Bao et al., "Optical properties of rotationally twinned InP NW heterostructures," Nano Letters, vol. 8, no. 3, pp. 836-841, 2008.[25] K. Pemasiri et al., "Carrier Dynamics and Quantum Confinement in type II ZB-WZ InP NW Homostructures," Nano Letters, vol. 9, no.
2, pp. 648-654, 2009.[26] N. Anttu, V. Dagyte, X. Zeng, G. Otnes, and M. Borgstrom, "Absorption and transmission of light in III-V NW arrays for tandem solar
cell applications," Nanotechnology, vol. 28, no. 20, p. 205203, 2017.[27] F. F. Yang et al., "Zn-doping of GaAs NWs grown by Aerotaxy," Journal of Crystal Growth, vol. 414, pp. 181-186, 2015.[28] W. Metaferia et al., "n-Type doping of Aerotaxy-grown GaAs NWs," Manuscript in preparation, 2017.[29] E. Barrigón, O. Hultin, D. Lindgren, M. H. Magnusson, L. Samuelson, and L. Johansson, "Aerotaxy GaAs NW pn-junctions,"
Manuscript in preparation, 2017.[30] W. Metaferia et al., "GaAsP NWs Grown by Aerotaxy," Nano Letters, vol. 16, no. 9, pp. 5701-5707, 2016.[31] R. E. Algra et al., "Twinning superlattices in indium phosphide NWs," Nature, vol. 456, no. 7220, pp. 369-372, 2008.[32] J. Wallentin, M. Ek, L. R. Wallenberg, L. Samuelson, K. Deppert, and M. T. Borgström, "Changes in Contact Angle of Seed Particle
Correlated with Increased Zincblende Formation in Doped InP NWs," Nano Letters, vol. 10, no. 12, pp. 4807-4812, 2010.
